Methods are known in which a worked piece is displaced by rotation and is then shaped from without by means of rollers or cylinders placed into contact with the worked piece. These worked pieces are often in the form of rotation-symmetrical hollow bodies. These hollow bodies may include floors closed from one side.
In most cases, this shaping is performed in a manner such that the outer shaping cylinders press the casing of the hollow-body shaped worked piece against an inner mandrel so that the material is placed into a shaping zone between the shaping-tool axially, radially, and tangentially to its rotation. This reduces the wall thickness of the casing.
The hollow body to be shaped is thus placed throughout its entire length onto the inner mandrel such that at least 50% of the finished worked piece is located on the inner mandrel after shaping. Dependent on the process, 100% of the length of the worked piece may be located on the inner mandrel.
Pressing the material against the inner mandrel during shaping includes the disadvantage that a high amount of friction arises between the inner surface of the worked piece and the surface of the mandrel. This friction leads in turn to disadvantageous heating of worked piece and inner mandrel. Also, wear to the surface occurs during the shaping process because of the friction load.
Furthermore, the ideal outer diameter of the inner mandrel must be determined experimentally, which requires time-consuming subsequent correction to the outer diameter of the inner mandrel. The option of manufacturing worked pieces with various inner diameters with various wall thicknesses within a worked piece by means of an inner mandrel is excluded. Production of such worked pieces requires a multi-piece inner mandrel, with parts attached to the main spindle and to the tailstock spindle. Such a device with a multi-part inner mandrel is expensive, however. Additionally, the potential contour of the worked piece is also limited in that a multi-part inner mandrel allows for only limited space provided for shaping the interior of the worked piece to be shaped.